Nanoparticles are microscopic particles that are less than 100 nanometers (nm) in at least one dimension. They are often referred to as “artificial atoms” because of their unique physical properties that allow them to behave similar to atoms (Alivisatos, A. P., Science, 1996, 271:933; Banin, U. et al., Nature, 1999, 400:542; Yu, D. et al., Science, 2003, 300:1277). The ability to assemble nanoparticles into desired nanoscopic architectures, such as nanoparticle superlattices, may result in a new way to fabricate functional materials of interest for a wide variety of practical applications, including biomedical diagnosis (Mirkin, C. A. et al., Nature, 1996, 382:607; Han, M. et al., Nature Biotechnol., 2001, 19:631; Sonnichsen, C. et al., Nature Biotechnol., 2005, 23:741), catalysis (Somorjai, G. A. et al., in Clusters and Nano-Assemblies, Jena, P. et al., Eds., World Scientific Publishing, New Jersey, 2005, pp. 97-125), plasmonics (Ozbay, E., Science, 2006, 311:189; Van Duyne, R. P., Science, 2004, 306:985), and high-density data storage (Sun, S. et al., Science, 2000, 287:1989).
Molecular self-assembly is the assembly of molecules without guidance or management from an outside source. Self-assembly often refers to the ability of molecules to form supramolecular assemblies. A simple example is the formation of a micelle by surfactant molecules in solution. Many biological systems use self-assembly to assemble various molecules and structures. Imitating these strategies and creating novel molecules with the ability to self-assemble into supramolecular assemblies is an important technique in nanotechnology. In self-assembly, the final structure is often “encoded” in the shape and properties of the molecules that are used, as compared to traditional techniques, such as lithography, in which the desired final structure must be effectively “carved out” from a larger block of matter. Self-assembly is thus referred to as a “bottom-up” manufacturing technique, as compared to lithography being a “top-down” technique.
Current methods for the assembly of nanoparticle superlattices have been based on controlling driving forces such as hard-sphere entropic depletion, electrostatic forces, and van der Waals, steric, and dipolar interparticle interactions (Rabani, E. et al., Nature, 2003, 426:271; Velev, O. D., Science, 2006, 312:376). These methods have led to the preparation of nanoparticle thin films and colloidal crystals with a variety of superlattice structures (Velev, O. D., Science, 2006, 312:376; Kiely, C. J. et al., Nature, 1998, 396:444; Murray, C. B. et al., Science, 1995, 270:1335; Saunders, A. E. and Kirgel, B. A., ChemPhysChem, 2005, 6:61; Shevchenko, E. V. et al., Nature, 2006, 439:55; Kalsin, A. M. et al., Science, 2006, 312:420), from which new collective properties have been discovered (Collier, C. P. et al., Science, 1997, 277:1978; Courty, A. et al., Nature Mater., 2005, 4:395). However, size- and shape-controlled nanoparticle superlattices have yet to be synthesized. These supercrystalline “superparticles” made of artificial atoms would be very useful in many industries.
Typical approaches for attempting to control the size and shape of nanoparticle assemblies include spray drying (Iskandar, F. et al., J. Colloid Interface 2003, 265:296), emulsion polymerization (Xu, H. et al., J. Am. Chem. Soc., 2006, 128:15582), mediator-induced assembly (Boal, A. K. et al., Nature, 2000, 404:746; Elussain, I. et al., Langmuir, 2006, 22:2938; Maye, M. M. et al., J. Am. Chem. Soc., 2005, 127:1519), and Debye screening (Kalsin, A. M. and Grzybowski, B. A., Nano Lett., 2007, 7:1018). Nanoparticle assemblies made by these existing approaches are susceptible to problems, such as poor size-distributions, poor shape-distributions, and a lack of long-range ordering (Iskandar, F. et al., J. Colloid Interface Sci., 2003, 265:296; Xu, H. et al., J. Am. Chem. Soc., 2006, 128:15582; Boal, A. K. et al., Nature, 2000, 404:746; Hussain, I. et al., Langmuir, 2006, 22:2938; Maye, M. M. et al., J. Am. Chem. Soc., 2005, 127:1519; Kalsin, A. M. and Grzybowski, B. A., Nano Lett., 2007, 7:1018).
Thus, it would be advantageous to have available a method of synthesizing nanoparticle assemblies that permits efficient control of size- and shape-distributions and long-range ordering.